Wireless microcontrollers

there is an ongoing revolution in wireless sensor networks. This has been driven by recent standardisation activities, namely the IEEE802.15.4 PHY/ MAC and the ZigBee network and application layers.

As seen in many other markets, particularly GSM, Wireless LAN and Bluetooth, the existence of the standard allows sales volumes to grow as manufacturers and consumers can rely on good performance and interoperability, giving a high level of confidence in the market.

With this background, many product manufacturers are ready to capitalise on wireless sensing and control technology, and to implement products using it.

Product examples include lighting systems, supply chain management, retail systems, wireless control and building management.

The common feature of all these applications is that they are all based on microcontrollers using standards based wireless connectivity.

One of the major factors influencing a successful product design is the availability of the tools needed for product design. In other words, manufacturers need to be able to design products easily.

The tools required range from evaluation/development kits to software development environments, protocol stacks, RF reference designs and manufacturing information packages.

Jennic's JN5121 single chip wireless microcontroller for IEEE802.15.4 and ZigBee applications is a good example of a chip platform that enables low cost, low power solutions to be realised. It includes a power efficient 32-bit CPU to implement both the protocol stack and the application, with both ROM and RAM memory on chip.

The CPU is accompanied by a comprehensive set of peripherals including analogue to digital and digital to analogue conversion, comparators, timers, SPI, IIC and UART interfaces and general purpose digital IO.

The chip also integrates the standards specific digital system and a radio system with a simple interface to the antenna or RF connector. The chip has been designed to allow end applications to be realised as easily as possible, using a minimum of design effort.

However, even with a highly integrated single chip solution like this, a range of tools and documentation needs to be supplied to ensure that the product design process is as simple as possible. To illustrate this, let us consider a simple development path, working from an initial proof of concept, through product prototyping to the final product design.

Proof of concept:

The first stage of development involves proving the product concept with a minimum of investment of time and money. Once a suitable chip has been chosen, a useful platform for product proving is often the device evaluation kit, provided that it includes some basic features.

Since this is a standards based design, there are many similarities between the requirements of different applications.

Therefore, the components of the evaluation kit should reflect the types of application that the device will be used in. Most importantly, it should be rapidly adaptable for the application, with a target of getting a first pass Evaluation kit boards demonstration application up and running in just a few hours.

For sensing and control applications, the kit needs a few simple sensors, switches and LEDs to enable simple applications to be developed without any additional hardware. Two types of boards are required: one that is small with a minimum feature set for the sensor application, and a second with a display to allow user friendly presentation of sensor measurements, or construction of simple user interfaces. If this hardware proves insufficient, all of the interface ports of the wireless microcontroller chip are connected to an expansion connector. This enables additional switches, sensors, data interfaces, etc, to be connected, providing the possibility of generating a fully operating mock-up of the end product.

This can allow the product concept to be proven without the need to go through lengthy board spins, RF layouts etc.

Whilst the hardware side of prototyping can be met by boards from an evaluation kit, an alternative approach is needed for the software development. This requires the availability of standard software development tools, including compiler, debugger, linker and, of course, a comprehensive reference manual.

As with the hardware, the availability of a comprehensive reference library of example applications helps the developer get started. For example, a standard home automation application that monitors sensor values and enables them to be plotted graphically or to trigger control events as they reach set values provides many of the key features of a wireless sensor network.

The examples can be modified quickly and easily to realise the initial prototype systems. As the market for standards based wireless control and sensing systems is so diverse, a range of protocol stacks, from simple point to point or star networks just using IEEE802.15.4 through to full ZigBee Protocol stacks is required.

For simple point-to-point or star based systems, the MAC software defined by IEEE802.15.4 is sufficient. For closed systems requiring coverage of wide areas, mesh or tree networking is required, and can be sourced from the network stack defined in the ZigBee standard.

For fully interoperable networks, the full ZigBee stack including security and profiles is necessary. In each case, the stack is accessed through an API, providing an easy to use interface to the application.

The choice of the most appropriate source for the stack will depend on many factors, including geographic location and support availability, and the type of application already integrated by the provider.

The standard ensures that the developer has access to a wide range of protocol features (security, acknowledge & retries, beacons, etc) that can all be used in the execution of the product design.

This means that the product designer can concentrate on his product design, secure in the knowledge that the network design issues have been taken care of.

Their main concern is then to interface the application (developed using the software developer kit, and using application examples) with the API at the top of the network stack.

With these tools and the evaluation kit hardware, developing the first prototype is as easy with a wireless microcontroller as it is with a conventional microcontroller development kit and, in many cases, prototypes can be up and running in less than 2 days.

Prototype development:

Having proven a product concept, the next step is to produce representative prototype hardware so that it can be more realistically demonstrated in its final physical form.

There are two key items that support product developers in this exercise: i) Modules and ii) Reference designs.

The availability of a module with full access to all of the chip IO pins provides a cost effective and easy method of realising a ready made RF solution.

The module is close to the smallest form factor that can be achieved, so enables the majority of applications to be realised close to their final size.

The use of the module at this stage has several benefits: i) it ensures that manufacturers do not need to spend time on RF board development and debugging; ii) since the modules are available off the shelf, the product can be developed in much shorter time than required to develop a custom RF board; iii) the devices are pre-tested, so it is not necessary to spend development time preparing an RF test application; iv) unless the application is to run in extremely high volumes, the module achieves economies of scale in manufacture and can offer a cheaper solution.

Hence, there are many benefits to the use of modules as part of the prototyping process–many of these extend to final production as well.

Since the module uses the same basic hardware as the evaluation kit, the same software development route can be used, allowing incremental changes to the software to be made as the development progresses.

For manufacturers who wish to integrate the chips onto their own boards to achieve the minimum size, or the very best costs, the availability of reference designs, which provide detailed information on the board layout (example board Gerber files, test solutions, etc), are key to success.

By implementing reference board layouts that have been proven in volume production as part of a product design, the manufacturer can proceed with confidence and reduced time to market as the number of board iterations can be reduced.

An important part of the reference design is the reference test system. This is a key area of RF system manufacturing that is often overlooked during the early part of the project.

For example, with the availability of test software, "golden" modules, and reference test hardware, a manufacturer can implement the RF test system as part of their production flow with very little engineering effort and be confident that the issues of designing such a system have already been taken care of.

The production RF test system is required to test parameters of the system that either cannot be tested as part of the chip test, or that are affected by the mounting of the chip onto the board.

A typical test system might include measurement of system output power, sensitivity, centre frequency and power consumption. The easiest way to implement a thorough test system is to use a “golden module” as a reference test system.

This module is carefully chosen to have significantly better performance than average, ensuring that the testing actually measures the device under test and not the test system.

By introducing attenuators between the golden unit and the device under test, measurements of the Packet Error Rate (PER) can be made at the defined sensitivity level.

The device under test can be either directly connected in a test jig, or through a reference antenna, depending on the type of product. A power meter can measure the output power of the system, after calibration for the duty cycle.

The centre frequency can be checked by monitoring the crystal frequency–if this is accurate and the device successfully transmits and receives, then it can be safely assumed that it is operating within the specified ±40ppm.

Another often overlooked requirement of product development is the need to comply with regulatory requirements. This can be a significant feature of the cost of development of a low volume product and is often a driving feature of the decision to use modules.

Using a pre tested module allows regulatory requirements to be met by reusing test results.

For example, in Europe there is a requirement for an intentional RF radiator to comply with the requirements of one of the harmonised standards issued to cover this type of product.

This testing is required in addition to the basic emissions and immunity testing. The end product manufacturer can use the appropriate test report from the module manufacturer as evidence that the product complies with the harmonised standard, hence removing the need to perform that particular test.

In USA, it is possible to obtain approvals for a module product on its own which can mean that an end product needs no further testing for compliance under its FCC part 15 rules.

By using existing modules or reference designs and adding the software developed using a software developer kit, the manufacturer can confidently complete the final product development with the minimum of development effort in the shortest time.

Summary:

The availability of an effective development tool chain has considerable impact on the effectiveness of end product development.

By providing effective development kits, protocol stacks, software developer kits, modules and production reference designs, the chip supplier can ensure that product development can be executed as quickly and easily as possible by allowing designers to focus on the product specific areas of the design. This enables product manufacturers to get operational product to market as quickly as possible.

Colin Faulkner is currently the Business Development Manager for ZigBee products at Jennic headquarters in the UK. Jennic ZigBee and IEEE 804.15.2 microcontroller, modules and evaluation kits are available in Australia and New Zealand from GLYN High-Tech Distribution. For more information about Jennic products, please visit GLYN’s website at www.glyn.com.au, or send your email enquiry to sales@glyn.com.au.

19-Feb-2007